scopas will do fine in there, get a small one. They are excellent coral guardians for algae. mine knows which pieces i put in that are new, and immediately inspects them for algae.
just remove him when hes to big. IMHO the Tang police are way to cynical. Keeping ANY fish in a tank is "cruel, by their same measure", when they have the whole reef to play on.
Benefits of tangs being in a tank far outweigh surviving on the reef.. Unlimited supply of food, no predators, no bully fish, dont have to worry about getting eaten, disease free.. ect..
A small wrasse like a sixline is fiine, but do be warned, just after about a year in the tank they tend to get pretty arrogant if a newcomer is added.